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Ultrasonic Transducers from Thermal Sprayed Lead-free

Piezoelectric Ceramic Coatings for In-situ Structural Monitoring for

Pipelines

Shifeng Guo, Lei Zhang, Shuting Chen, Chee Kiang Ivan Tan, and Kui Yao*

Institute of Materials Research and Engineering, A*STAR (Agency for Science,

Technology and Research), #08-03, 2 Fusionopolis Way, Innovis, Singapore 138634

ABSTRACT

Direct-write ultrasonic transducers comprising of lead-free ceramic coatings were designed and

in-situ fabricated by thermal spray process onto stainless steel pipe structures. The experimental

measurements and simulation analyses showed that both external and internal defects on the

pipe structures could be effectively detected with the ultrasonic Lamb waves propagating along

the axial direction, which were excited and detected using the direct-write ultrasonic transducers.

The results suggested that the implementation of the direct-write ultrasonic transducers made of

the lead-free piezoelectric ceramic coating by thermal spray method is promising for realizing

structural health monitoring for pipeline structures with improved consistency and reliability. It is

also an important step moving toward smart pipelines with real-time self-diagnostic function.

Key-words: Direct write, Ultrasonic transducer, Lead-free, Piezoelectric ceramic, Thermal spray,

Pipe structure, Lamb wave, Structural health monitoring

* Corresponding author. E-mail contact: k-yao@imre.a-star.edu.sg

The revised version published: Shifeng Guo, Lei Zhang, Shuting Chen, Chee Kiang Ivan Tan, and Kui Yao, “Ultrasonic Transducers from Thermal Sprayed Lead-free Piezoelectric Ceramic Coatings for In-situ Structural Monitoring for Pipelines,” Smart Materials and Structures, https://doi.org/10.1088/1361-665X/ab1e88, Vol. 28, No. 7, 075031, 2019.

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I. INTRODUCTION

Metallic pipe structures are widely utilized in oil and gas, marine and offshore, chemical and

refinery industries, and many other utility infrastructures. The corrosion, pitting, or crack defects

in such pipe structures can lead to catastrophic consequences if not detected in advance,

therefore the effective and reliable non-destructive testing (NDT) techniques are highly demanded

[1, 2].

Various techniques have been applied for inspecting the integrity of pipe structures, such as

visual inspection, ultrasonic techniques, radiography, electromagnetic techniques and eddy

current techniques. Ultrasonic guided waves have received particular attention in metallic pipe

inspection in comparison with other NDT methods because of their capabilities of long distance

propagation and deep penetration [3-6].

Guided waves are often generated and detected with discrete ultrasonic transducers, which are

conventionally integrated with additional wedges under external pressure or using acoustic

couplant for inspection of the pipe structures [7, 8]. However, the use of wedge transducers not

only increases the assembling complexity, but also generates unwanted wave forms and

inconsistency related to the location, pressure, and alignment. Besides the discrete angle beam

transducers, piezoelectric ceramic patches or discrete ultrasonic transducers are assembled or

bonded on the pipe structures acting as both actuators and sensors for defects detection [9-11].

In some cases, the interdigital comb transducers, made of piezoelectric ceramics or polymers,

were bonded/attached on the structures to excite and detect the guided wave for structural health

monitoring (SHM) purpose [12-14]. However, the introduction of external bonding process or

fixture of ultrasonic transducers has issues of inconsistency and unreliability due to manual

installation, and also results in high cost with the implementation of a large number of sensors.

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Therefore, an in-situ, reliable, and low-cost ultrasonic transducer implementation technology

with capability of monitoring structural failure occurrences and detecting the external and internal

structural defects, over a long axial distance, is greatly demanded for pipeline SHM. In contrast

to existing ultrasonic NDT techniques that utilize conventional discrete acoustic transducers,

either handheld or installed manually on the structures with extra fixtures, adhesive or acoustic

couplant, the poly(vinylidene fluoride/trifluoroethylene (P(VDF-TrFE)) polymer based direct-write

transducers, in which the piezoelectric polymer films and electrodes are directly deposited and

patterned on the structures as concerned, were recently developed to detect both micro and

macro scale defects on plate structures [15-17].

While great promise has been exhibited with the direct-write piezoelectric polymer ultrasonic

transducers for realizing real-time in-situ SHM, including many potential advantages including

improved consistency, and potentially lowered cost for implementation over a large area, the

polymer ultrasonic transducers have some intrinsic limitations due to relatively low piezoelectric

and electromechanical coupling coefficients, susceptible to external load or impact, and not

applicable in high temperature conditions.

In order to develop more robust and high performance direct-write ultrasonic transducers, the

idea of using potassium sodium niobate (KNN) based lead-free piezoelectric ceramic ultrasonic

transducers to generate and receive the guided waves for inspecting the defects of pipe structures

is proposed in this work. The KNN-based ceramics are considered one of the most extensively

investigated lead-free systems because of the large piezoelectric coefficient and high Curie

temperature [18, 19]. Recently, thermal-sprayed KNN-based lead-free piezoelectric ceramic

coating on metallic structures, which exhibited single phase of perovskite structure and strong

piezoelectric response in comparison with the polymer based piezoelectric films, were reported

by our group [20-22]. The lead-free KNN-based ceramic compositions are environmentally

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friendly and have significantly enhanced acoustic coupling with metallic structures, and thus are

promising for large-area SHM applications.

In this work, the fabrication of KNN-based lead-free piezoelectric ceramic coatings on stainless

steel pipe structures and an innovative pipe SHM method using the KNN-based lead-free

piezoelectric ceramic coatings and electrodes, which are both in-situ deposited and patterned on

the pipe structure, are explored. Lamb mode ultrasonic waves propagating along the pipes are

generated and detected by the KNN-based direct-write transducers for analyzing the structural

integrity, in which both external and internal defects of the pipe structures are detected. The

results show KNN-based direct-write transducers have the potential for achieving effective

pipeline SHM.

II. METHOD AND EXPERIMENTAL DETAILS

A. KNN-LiTaSb Powder Preparation and Coating Deposition

K2CO3 (99.5 %), Na2CO3 (99.0 %), Nb2O5 (99.9 %), Li2CO3 (99.999 %), Ta2O5 (99.85 %), Sb2O5

(99.998 %) powders (Alfa Aesar, Karlsruhe, Germany) were used to produce the piezoelectric

ceramic powder with stoichiometric composition of (K0.44Na0.52Li0.04)(Nb0.84Ta0.10Sb0.06)O3 (KNN-

LiTaSb) [20, 21]. To compensate the loss of K and Na during the high temperature processing,

extra 10 mol% of K and Na were added. The detailed process of the KNN-powder was introduced

in published reports [20, 21]. In short, the weighed materials were mixed with ethanol and zirconia

balls in a planetary ball mill for 24 hours. The slurry was subsequently dried and crushed before

it was calcined at 1000 oC for 5 hours in an alumina crucible. The calcined ceramic powder was

further crushed and then fractionated for thermal spray process.

A stainless steel (316L) pipe measuring 190 mm in length, 33 mm in outer diameter and 3.5

mm in thickness, was selected for SHM method demonstration. Two intermediate layers: a

67%Ni-22%Cr-10%Al-1%Y (NiCrAlY) bond coat layer with a thickness of 50 μm, and a 8 mol%

yttria-stabilized zirconia (YSZ) thermal barrier layer with a thickness of 200 μm were deposited

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on the outer surfaces of pipes by an atmospheric plasma spraying (APS) system [Fig.1(a)]. The

intermediate layers serve to reduce the internal stress of the structure by mitigating thermal

mismatch among the multiple layers, eliminate steel oxidation by forming oxygen barrier layers,

and hence improve the sustainability of the structure at high processing temperature and enhance

the piezoelectric performance property [20].

An electrode layer of Ag-Pd (70/30) was subsequently deposited on the outer surfaces of the

pipes and heat-treated to serve as bottom electrode [Fig. 1(b)]. The KNN-LiTaSb powder was

thermal sprayed onto the selected regions of the pipes using the same APS system to have the

coatings of 120 μm in thickness [Fig. 1(c)]. To enhance the crystallinity and electrical properties,

the KNN-LiTaSb coating was annealed at 950 oC for 30 mins. The comb electrodes were

patterned on the pipe and the KNN-LiTaSb coating was electrically polarized to obtain

piezoelectric property for subsequent ultrasonic testing [Fig. 1(d)]. The periodicity of the comb

electrode pattern was designed to match the wavelength of the selected acoustic wave mode.

Fig. 1. Fabrication of piezoelectric ceramic transducers on stainless steel pipe structure. (a) Stainless steel

pipes (SS316) with artificial defects formed and two intermediate layers: a NiCrAlY bond coat layer and a

thermal barrier YSZ layer. (b) A stainless steel pipe coated with two bottom electrodes of 70Ag/30Pd. (c)

Pipes after the deposition of KNN-LiTaSb coatings with thermal spray. (d) A stainless steel pipe after the

fabrication of interdigital top electrodes.

B. Dispersion Curves of the Stainless Steel Pipe Structures

To excite the effective guided waves for defects detection of the pipe structures, an optimal

ultrasonic mode and frequency are highly demanded. It is important to generate a specific guided

wave of which wave propagation characteristics are predetermined. Therefore, the dispersion

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curves, of which the propagation characteristics such as wave velocity and wavelength are

important for the selection of correct mode. The dispersion curves of the pipe were simulated for

the design of direct-write transducers as shown in Fig. 2. It is desirable to select the fast mode as

this will facilitate the time domain separation of the signals of interest [10]. In consideration of both

wavelength and pipe dimension, the asymmetric wave mode L(0,5) at frequency of 830 kHz was

pre-selected to design the comb transducers [Fig .2(b)]. The details of the design parameters of

the direct-write transducers are provided in Table 1. With the same wavelength at 830 kHz of 7.0

mm, the other asymmetric modes as [L(0,4), 733 KHz], [L(0,3), 606 kHz], [L(0,2), 576 kHz],

[L(0,1), 350 kHz] will be excited and compared [Fig. 1(a)].

Fig. 2. Dispersion curves of the stainless steel pipe structure. (a) The phase velocity versus frequency. (b)

The group velocity versus frequency.

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Table 1. The design parameters of the direct-write transducers integrated on the pipe structure using

asymmetric L(0, 5) mode at 830 KHz.

C. Numerical Simulation

The 2D axial finite element simulations were conducted with ANSYS (v. 19.2) to evaluate the

design of direct-write transducers for defect detection in metallic pipes. Coupled field element

PLANE223 was used to simulate the piezoelectric effect in the direct-write transducers. Figure

3(a) illustrates the axial symmetrical model with an external notch of 1 mm in depth and 7 mm in

length. A pulse signal as shown in Fig. 3(b) is applied to the top transducer and the broadband

ultrasonic waves are generated. The ultrasonic waves propagate from the top transducer to the

bottom transducer. Figure 3(c) shows the time domain signals for conditions of with and without

the external notch. It is evident that the ultrasonic waves are affected by the defect. Figure 3(d)

reflects the difference of sensor signals for the pipe with and without defect in the frequency

domain, showing that the difference in sensor signals is larger in the frequency range of 200-500

kHz and 700-900 kHz. These frequency ranges will be examined experimentally together with the

dispersion relation shown in Fig. 2(a) to find the optimal testing frequency for defect detection.

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Fig. 3. (a) The 2D axial symmetrical model of pipe with direct-write transducers. (b) Electrical pulse signal

applied to the transducer working as actuator. (c) Time-domain signals from transducer as sensor for pipe

with and without a 1-mm deep external defect. (d) Frequency domain signal difference for pipe with and

without the 1-mm deep external defect.

D. Design and Fabrication of Direct-write Transducers for Axial Lamb Wave on Pipe Structure

The schematic of three pairs of piezoelectric ultrasonic transducers of same size, each made

of the KNN-LiTaSb coating deposited on the stainless steel pipe is shown in Fig. 4(a). For each

pair of the transducers, one of the transducers at one end worked as the actuator for generating

guided Lamb acoustic waves propagating along the axial direction, and the other transducer at

the opposite end served as the sensor for receiving the acoustic waves. Any defects in the

propagation path between the actuator and the sensor could be detected by the change of

acoustic signals.

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Fig. 4. (a) The schematic of one pair of direct-write transducers on defect detection of pipe structure. (b-c) The cross-sectional views of the pipe structure showing the external and internal defects introduced on the pipe. (d) The dimensions of the defects. Here L, W, D indicate length, width, and depth, respectively.

Each of the comb transducers has three equally spaced comb fingers with the periodicity of

electrode fingers equal to the wavelength of the selected guided waves for detecting defects [13].

In order to investigate the performance of KNN-LiTaSb direct-write transducers on defects

detection, the artificial defects with different dimensions were introduced on the external surfaces

of the pipe to simulate levels of damages as illustrated in Fig. 4(b). The ultrasonic signals obtained

from the structures with and without defects were analyzed and compared. Three levels of

integrity were inspected including Control (without defect), Defect I and Defect II, with the three

pairs of transducers fabricated along the circumferential direction. Furthermore, to investigate the

capability of direct-write transducers on the detection of internal defects, the ultrasonic signals

from Control (without internal defects) were firstly recorded. The internal defects were

subsequently introduced between the actuator and sensor, and the defects were detected by

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comparing the ultrasonic signals before and after defect. The detailed dimensions of the defects

were given in Fig. 4(d).

Damage detection in pipe structure utilizes a baseline dependent method [9] for determining

the damage state, with comparison of ultrasonic signals from structures with and without defects,

the presence of defects can be identified. By tracking the degree of ultrasonic signal change, it is

possible to monitor the growth or size of the damage. During experiment, a Hanning-windowed

signal, with 3.5 cycles from the function generator (Agilent 33210A, USA) was fed into a high

speed linear power amplifier (EI2100L, USA) to drive the actuator. The ultrasonic wave that

propagates along the pipe structure is detected by the sensor. When the propagating ultrasonic

wave encounters a discontinuity/defect in the structure, the wave is reflected or scattered. The

differences among the sets of signals are indicative of damage level.

III. RESULTS AND DISCUSSION

A. Characterization of KNN-LiTaSb based Direct-write Transducers

Figure 5 presents a photo of the stainless steel pipe under testing with in-situ fabricated KNN-

LiTaSb direct-write transducers and artificial external defects. To fabricate the top electrode with

comb pattern, silver paste were brushed on the KNN-based piezoelectric coating with the aid of

shadow mask as shown in Fig. 5(a). The periodicity of the comb electrode pattern matched with

the wavelength of the selected Lamb wave mode [(830 KHz, L(0,5)] of 7 mm. The direct-write

transducers were fabricated along the circumferential and axial direction of the pipe structure,

where one transducer in one of the three transducer pairs served as an actuator for generating

ultrasonic wave propagating along the axial direction and the other transducer served as a sensor

for receiving the ultrasonic wave. Any defects in the propagation path between the actuator and

the sensor would affect the signals of the ultrasonic waves and thus the integrity of pipe could be

monitored.

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The effective piezoelectric coefficient (d33) of the thermal sprayed KNN-LiTaSb coatings was

measured using a laser scanning vibrometer (LSV), which is a reliable method for determining

the piezoelectric longitudinal strain coefficient for piezoelectric coating on a substrate [23] . With

a unipolar 1 kHz AC voltage of 20 V applied onto the transducer, an area covering the regions

with and without electrode were scanned as shown in Fig. 5(b) and the effective piezoelectric

coefficient d33 value of the KNN-LiTaSb coatings on the pipe was determined as ~40 pm/V under

the in-plane mechanical clamping condition [Fig. 5(c)]. The ultrasonic testing was conducted with

the KNN-LiTaSb direct-write transducers, acting as both actuators and sensors, as schematically

illustrated in Fig. 5(c). A tone Hanning windowed ultrasonic signal from a function generator was

amplified by a linear power amplifier to drive the actuator, and the ultrasonic wave propagated

along the pipe structure was received by the sensor. A representative ultrasonic signal received

by the direct-write sensor is shown in Fig. 5(e), and the first-arrival ultrasonic signals were

compared to evaluate the defects.

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Fig. 5. Piezoelectric KNN-LiTaSb direct-write transducers in-situ fabricated on a stainless steel pipe for

defect detection. (a) Photo of the stainless steel pipe with in-situ fabricated direct-write transducers and

artificial defects. (b-c) The piezoelectric coefficient (d33) characterization of the direct-write transducers

using laser scanning vibrometer (LSV). (d) Set-up for generating and detecting ultrasonic signals by the

direct-write transducers. (e) The representative time domain ultrasonic signal detected by the direct-write

transducers.

B. The External and Internal Defects Detection with Axial Lamb Wave on Pipe Structure

During testing, the actuator was excited by a 3.5-cycle Hanning windowed signal and the

ultrasonic signal received by the sensor was collected with an oscilloscope and analyzed. To

identify the optimal frequency for the subsequent ultrasonic analysis, different modes at

frequencies of 350 kHz, 576 kHz, 606 kHz, 733 kHz and 830 kHz with the same wavelength of 7

mm were explored [Fig. 1(a)]. At the design frequency of 830 kHz, the received signals were

overlapped and not distinguishable as many other modes at this frequency with similar group

velocity were excited, as shown Fig. 6(f), and therefore difficult for signal analysis. Similar

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phenomenon was observed for frequency of 733 kHz. Among all the frequencies, 350 kHz was

the optimal frequency considering the relatively large signal amplitude and ease of mode

separation for signal analysis. Therefore, the driving frequency of 350 kHz was selected for

demonstrating the capability of KNN-LiTaSb direct-write transducers on defect detection,

including both the external and internal defects.

Fig. 6. The comparison of ultrasonic signals among different frequencies with the same wavelength of 7

mm. (a) The schematic of direct-write transducers on stainless steel pipe structure for defect detection. (b-

f) Signals of different modes and frequencies for comparison with same wavelength of 7 mm.

The performance of the KNN-LiTaSb direct-write transducer as designed was also investigated

using the scanning mode between 100 kHz to 1 MHz, with the frequency interval of 10 kHz. The

results show that in the range of 260 - 450 kHz and 600 - 950 kHz, the direct-write transducer

could generate strong ultrasonic signals, which is consistent with simulation results as shown in

Fig. 3(d). The regions marked in Fig. 7 show the first arrival signals, where signals at 350 kHz

were selected for amplitude comparison with and without defects.

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Fig. 7. Ultrasonic signals in comparison from pipe structure with different defects, in the range of 100 kHz

to 1 MHz. (a) The Control signal response from pipe without defect. (b) The pipe with 1-mm external defect.

(c) The pipe with 2.5-mm external defect. The marked regions indicate the first arrival signals.

For the Control as shown in Fig. 8, the measured group velocity based on the time of flight for

the asymmetric mode L(0, 1) at 350 kHz is 2940 m/s, which is consistent with the theoretically

calculated group velocity. It is used for the comparison with the same mode signals observed for

the pipes with external Defects I and II as shown in Fig. 8 (a).

The presence of the simulated external artificial defects led to substantial reduction of acoustic

amplitudes as shown in Fig. 8. The amplitude of Control signal was reduced by 36% with the

notch of 1-mm depth (Defect I), and further reduced by 60% with notch of 2.5-mm in depth. The

effect of the presence of external defects on the substantial reduction of ultrasonic signals were

reported in the previous study using commercial discrete ultrasonic transducers [4, 8, 9], in which

the ultrasonic transducers were bonded or attached on the pipes using external fixtures. However,

the direct-write transducers can determine the change with significantly improved comparability,

and thus high reliability, due to the highly consistent measurement condition without requiring

acoustic coupling agent, mechanical fixture, and any other repeatability issues induced by human

operation factors, such as pressure, location and alignment. The 2D axial finite element simulation

also indicated that the external defects led to substantial acoustic amplitude reduction with

increased external defects at 350 kHz (Fig. 8b). The difference between the simulation and

experimental results may be due to the irregular feature and fabrication tolerance of the actual

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defects in the experiment. It is also noted that in the simulation results, the defects lead to delay

of acoustic waves in time-domain, in addition to the reduction in signal amplitude. This may be

due to the change of wave velocity at the defect location. In the experimental fabrication, the three

sets of transducers, which the measurements were conducted, had location tolerance, thus

consistent delay of acoustic waves in time-domain was not captured.

Fig. 8. (a) Comparison of time-domain ultrasonic signals among pipe structures with and without external

defects: Control (blue line), 1-mm depth notch (red line), and 2.5-mm depth notch (black line). The marked

area represents the first-arrival signal and zoomed for ease of comparison. (b) Comparison of simulated

time-domain ultrasonic signals among pipe structures with and without external defects.

More importantly, it was further noted that internal defects that were not feasibly observed by

visual inspection was also detected with the KNN-LiTaSb direct-write transducers. An artificial

internal notch, with dimension of 20 mm (length) x 7 mm (width) x 2.5 mm (thickness) was

introduced in the inner surface of pipe as schematically shown in Fig. 2(b). The time domain signal

before and after the internal notch formed in the same pipe was recorded and compared in Fig. 9

(a). The result shows that the ultrasonic amplitude was reduced by 51% as a result of the internal

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defect, as shown in the marked area in Fig. 9. It is also noted that the internal defect led to delay

of acoustic waves in time-domain, in addition to the reduction in signal amplitude. This may be

explained by the change of wave velocity at the defect location. The time-domain signals were

further analyzed in frequency domain as presented in Fig. 9(b). Similar to the time domain signal,

the ultrasonic signal amplitude at the frequency of 350 kHz was reduced substantially due to the

internal defect. Simulation results in time domain and frequency domain for the internal defects

are shown in Fig. 9 (c-d). Both time and frequency domain results show the reduction of ultrasonic

signal due to the presence of internal defects. In the simulation results, the defect led to delay of

acoustic waves in time-domain, which is consistent with the experimental result. The results

further validate that the direct-write transducers are capable of exciting and receiving bulk

ultrasonic wave, and detecting both internal and external defects in the pipeline structures.

In addition to the detection of internal and external structural defects using guided waves, the

direct-write piezoelectric transducers can also be used for impact detection and localization for

subsea pipelines. Impacts account for 47% of subsea pipeline failures, and recently new method

for rapid localization of impacts on underwater pipeline structures has been developed with

discrete lead zirconate titanate (PZT) transducers [24, 25]. Considering the merits of direct-write

lead-free piezoelectric transducers over discrete PZT transducers, such as consistency, reliability,

and low cost for large area implementation, it is interesting to explore the value of application of

the direct-write green piezoelectric ceramic transducers for impact detection and localization.

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Fig. 9. (a) and (b) Time- and frequency-domain ultrasonic signals on the pipeline structures with and without

internal defect. (c) and (d) The simulated time- and frequency-domain signals with and without internal

defect.

IV. CONCLUSION

Direct-write piezoelectric ceramic ultrasonic transducers were designed and fabricated onto stainless

steel pipe structures for structural health monitoring. The piezoelectric ultrasonic transducers were made

of KNN-LiTaSb lead-free piezoelectric ceramic coatings deposited by thermal spray process and comb-

shaped electrodes by printing process for exciting and detecting Lamb ultrasonic wave propagating along

the axial direction of the pipes. The experimental measurements and simulation analyses showed that both

the external and internal defects could be effectively detected due to the substantial acoustic amplitude

reduction as a result of presence of the defects. The results suggested the implementation of the direct-

write ultrasonic transducers comprising of the lead-free piezoelectric ceramic coating is promising for

realizing structural health monitoring for pipeline structures with improved consistency and reliability, in

consideration of that it can determine the ultrasonic signal change with significantly improved comparability

and reliability. It is also an important step moving toward smart pipelines with real-time self-diagnostic

function.

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ACKNOWLEDGEMENTS

This project is partially supported by Singapore Maritime Institute under the Asset Integrity & Risk

Management (AIM) R&D Programme, Project ID: SMI-2015-OF-01 (IMRE/15-9P1115), and

under the Maritime Sustainability R&D Programme, Project ID: SMI-2015-MA-07 (IMRE/16-

7P1125).

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